(a) Technical Field
This disclosure relates to the field of semiconductor processing and, more specifically, to wafer boats used in vertical furnaces.
(b) Description of the Related Art
In semiconductor processing, vertical wafer boats are used to hold a plurality of semiconductor wafers in a vertically stacked spaced relationship for processing in a vertical furnace. The wafer boats respectively comprise three or more vertical members which extend between a top member and a bottom member. The vertical members are provided with a plurality of vertically spaced recesses and the bottom surfaces of the recesses form support surfaces for the wafers. The recesses have a constant pitch over substantially an entire height of the wafer boat. The wafer boat is configured such that a wafer can be inserted into a wafer accommodation between two side vertical members from the front side, which is opposite the one or more back vertical members. The total number of wafers that can be processed simultaneously, or the load size, follows from the length of the furnace that provides a flat temperature profile that can be used for processing divided by the interval, or pitch at which the wafers are placed. In some processes the wafer pitch influences the process results, such as in CVD processes where the within wafer uniformity is influenced by the wafer pitch and the minimum usable pitch is determined by the process requirements. In other processes such as oxidation, anneal and curing processes the wafer pitch has limited or no influence on the process results and the minimum usable pitch is determined by the space that is required for loading and unloading the wafers into and from the wafer boat. The space required is determined by the thickness of the end effector, the bow of the end effector and the wafer, and the tolerances in the dimensions of the wafer boat and the accuracy of the wafer handling robot. With increasing wafer size, the weight of the wafer increases and the thickness of the end effector increases. Further, as the increase of the thickness of the wafer is less than proportional to the increase of the diameter of the wafer, the bow of the wafer increases. As a result, for 300 mm wafers and 450 mm wafers the minimum required handling space is significantly larger than for smaller wafers. Correspondingly, the total wafer load that can be accommodated in a furnace having a certain flat-zone length decreases.